1. Field of the Invention
The invention relates to communications systems in general, and, more particularly, to methods and apparatus for error control in communications systems which employ virtual circuit technology.
2. Background of the Invention
A communications system permits users to communicate over an extended distance and typically comprises a multiplicity of devices and information channels to facilitate the communication. Perhaps the most familiar and ubiquitous of all communications systems is the telephone system. This includes, for example, the local- and long-distance telephone networks, private networks and the private branch exchanges used to provide special services to businesses and institutions. While the telephone system carries information in both voice and data forms, similar computer networks carry data between computers.
FIG. 1 presents an illustrative communication system comprising a communication network 109 connecting two users, Alice 101 and Bob 103. When Alice desires to transmit information to Bob, she provides her system station 105 with the information. The system station may be any device (e.g., a telephone, facsimile machine or computer) which is capable of transforming the information a user provides to a form the communication system can handle, and vice versa. When the system station is an analog telephone, the system station transforms the acoustic vibrations of Alice's voice into an ordered set of signals representing analog quantities ("analog signals") for transmission over the communication system. When the system station is a digital telephone, facsimile machine or computer, the system station typically transforms the information into an ordered set of signals or "packet" representing a set of digital quantities ("digital signals").
After the system station 105 creates the set of signals, the system station transmits the signals, over a communication network 109, to Bob's system station 107 where Bob's system station transforms the signal into a form that is suitable for presentation to Bob.
A typical packet switching system is disclosed in U.S. Pat. Re. 31,319, by A. G. Fraser, reissued Jul. 19, 1983, and enabled communications systems to carry packets. Such packet-switched communications systems are well known in the prior art, carry both voice and non-voice data and generally are more economical and utile than their analog counterparts.
A packet-switched communication system can destroy a packet either by corrupting (i.e., changing the signals in) it or by losing all or a portion of the signals in it. A packet retains its "integrity" when it is delivered with no changed or lost signals. When a packet is destroyed during transmission, however, it becomes impossible to determine the integrity of the packet. Although merely determining whether a packet has been destroyed is problematic, once it is determined that the packet has been destroyed, the generally accepted procedure is to discard the packet and request retransmission by the sender. This is the practice followed by the TCP/IP protocol suite and such other well-known communications protocols as LAP/LAPB/LAPD, X.25/X.75 and Q.921/Q.931.
To determine whether a packet has been destroyed it is necessary to make certain assumptions about how the packet can be destroyed. Assuming that the signals within a packet may be changed but never lost, a Cyclic-Redundancy-Check will determine whether the integrity of the packet has been destroyed or not. These are the assumptions and the solution for many data networks (e.g., X.25, Ethernet, FDDI and Bisync).
Assuming that the signals within the packet can be lost but not changed, a packet length indicator and a packet sequence identifier will determine whether the integrity of the packet has been destroyed or not. This is the assumption and the solution used in Datakit networks marketed by AT&T, as implemented in the Universal Receiver Protocol disclosed in Fraser et al, U.S. Pat. No. 4,852,127, issued Jul. 25, 1989.
Packet-switching networks which employ "virtual circuit" technology allocate network resources such that all of the information sent across the network is sent successively in the order that the information is submitted to the network. Such virtual circuit packet-switching networks are well-known in the prior art, generally carry non-voice data and are more utile than their datagram counterparts.
Virtual circuit packet-switching networks can both lose signals within a packet and can corrupt the remainder. Therefore, it is not advisable to make any of the simplifying assumptions made for networks above. Thus, it has been problematic to devise a mechanism for determining whether the integrity of a packet has been destroyed or not in virtual circuits. Merely combining the mechanisms of a Cyclic-Redundancy-Check, a packet length indicator and a packet sequence identifier is insufficient.